Reply-To: "George Gonzalez"
From: "George Gonzalez"
References: <firstname.lastname@example.org> <email@example.com>
Subject: Re: hi-capacity voltage controlled capacitor?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
NNTP-Posting-Date: Wed, 09 Oct 2002 12:58:28 GMT
Organization: AT&T Broadband
Date: Wed, 09 Oct 2002 12:58:28 GMT
"transformer" wrote in message
> The truth is that I have built such a device but do not know if it is
> a useful invention.I see that there are no such devices present. Could
> anybody comment on its usage. The invented device varies between 2uF -
> 100nF and has bad linearity.
To the best of my knowledge, it's possible to simulate a large variable
capacitor, but they're generally not very good capacitors-- i.e. poor Q,
poor frequency response, low current/voltage capability, require power.
And it's not even remotely economical in most cases. Usually one goes to
almost any lengths to redesign the circuit to not need large variable
Some alternatives are: raise the circuit impedance levels so you can use
smaller capacitors, use relays to switch in capacitance, up-convert the
signal to a higher frequency, use PWM switched-capacitor methods, use
variable resistors instead,
use rotary-variable inductors instead, or use saturable-variable inductors.
One example is in computer monitors-- most of them nowdays can handle a wide
range of H sweep speeds-- this requires being able to tune the H deflection
coil over a 3-1 freq range, a 9-1 cap range. And this is in a high
voltage/current circuit. Most monitors do this (approximately) by switching
in several capacitors with several relays, or several hefty MOSFET switches
in the newer monitors. In theory it could be better done with very hefty
op-amps or motor-driven mica compression trimmers, but those are
impractically large, power-hungry and expensive. Switched relays do a "good
enuf" job for this application. I imageine there's some high-end HP or
Ikegami monitor for medical applications that uses some sort of
active-capacitor, but as usual the price is sky-high.